KR102531813B1 - High-strength stainless steel sheet with excellent fatigue properties and manufacturing method thereof - Google Patents

Abstract

Objective: To realize a thin plate material having a distribution form of non-metallic inclusions effective for improving fatigue resistance in a strain-induced martensite-forming type stainless steel with a high Si content.
Solution: In mass%, C: 0.010 to 0.200%, Si: more than 2.00% and not more than 4.00%, Mn: 0.01 to 3.00%, Ni: 3.00% or more and less than 10.00%, Cr: 11.00 to 20.00%, N: 0.010 to 0.010% 0.200%, Mo: 0 to 3.00%, Cu: 0 to 1.00%, Ti: 0 to 0.008%, Al: 0 to 0.008%, the balance consisting of Fe and unavoidable impurities, with a distance between particles in the rolling direction of 20 μm or less Also, among non-metallic inclusions with a distance between grains of 10 µm or less in the sheet thickness direction, the number density of those having a length of 40 µm or more in the rolling direction is 3.0 pieces/mm2 or less in the L cross section.

Description

High-strength stainless steel sheet with excellent fatigue properties and manufacturing method thereof

The present invention relates to a steel sheet in which formation of a strain-induced martensite phase, solid solution strengthening by high Si content, and very high strength can be obtained by using age hardening, in a stainless steel grade in which the formation of coarse hard nonmetallic inclusions is significantly suppressed. it's about It also relates to a manufacturing method thereof.

As high-strength stainless steel, metastable austenitic stainless steel typified by SUS301 is widely used. However, in order to obtain high strength with SUS301, it is necessary to increase the cold rolling rate, which is accompanied by a decrease in toughness. As a technique for avoiding this problem and achieving both high strength and high toughness at a high level, a technique for achieving high strength by using formation of a strain-induced martensite phase, solid solution strengthening by high Si content, and age hardening is known. , ID saw blade board, etc. are used (Patent Document 1).

Stainless steel of the type disclosed in Patent Literature 1 has a double phase structure of strain-induced martensite and austenite obtained by cold rolling, has high strength and high toughness, and is fatigue resistant as a rotary member having a plate thickness of 0.1 mm or more such as an ID saw blade. Characteristics are also good. However, for example, in the case of application to a spring material application that is processed into a thin plate material having a plate thickness of less than 0.1 mm, particularly 20 to 70 μm, and subjected to repeated elastic deformation, further improvement in fatigue resistance is desired. . As a factor deteriorating the fatigue resistance of steel materials, the presence of non-metallic inclusions may be mentioned. Even for inclusions of the same size, as the plate thickness decreases, the ratio of the length of the inclusion to the plate thickness in the thickness direction increases, and stress is concentrated around the inclusion particles, making it easier to function as a crack origin or propagation path. The thinner the plate, the more difficult it is to improve the fatigue property deterioration caused by non-metallic inclusions.

As a method of reducing the amount of non-metallic inclusions in steel materials (ie, increasing the cleanliness), various methods of optimizing the slag composition during refining have been studied. However, from the viewpoint of preventing work cracking and fatigue failure, simply improving the cleanliness is not necessarily sufficient, and it is considered effective to control the composition of non-metallic inclusions. For example, in Patent Document 2, the composition of non-metallic inclusions is controlled by adjusting the slag basicity to 1.4 to 2.4 using a refining furnace having a dolomite-based refractory lining in a general-purpose austenitic steel grade solvent such as SUS304, and processing cracking. A method for obtaining an austenitic stainless steel free of this is disclosed. However, according to the investigation by the inventors, it was found that in the case of a steel grade having a high Si content, it is difficult to significantly improve the fatigue properties of thin plate materials even if the method disclosed in Patent Document 2 is tried.

On the other hand, Patent Document 3 discloses a technique for improving corrosion resistance in high-temperature, high-concentration nitric acid by reducing the total amount of high-melting-point B ₁ -based inclusions mainly composed of alumina in high-Si austenitic stainless steel. In order to suppress the formation of _B1- based inclusions, it is described that reducing and recovering Cr with Al is not performed, and that an Fe—Si alloy having an Al content as low as 0.1% is added (paragraphs 0052 and 0053). However, the steel targeted in Patent Literature 3 is an austenite single-phase steel with a Ni content of 10% or more (paragraph 0033), and is not a steel type intended to increase strength by forming a strain-induced martensite phase. This document does not teach a method for improving the fatigue resistance properties of thin plate materials in spring applications. As will be described later, it is important to suppress the generation of TiN-based inclusions in order to improve the fatigue resistance of the steel type targeted in the present invention, but the melting method disclosed in this document cannot stably reduce TiN-based inclusions. .

Patent Document 1: Japanese Patent Publication No. 3219117 Patent Document 2: Japanese Patent Publication No. 3865853 Patent Document 3: Japanese Patent Publication No. 5212581

Among the non-metallic inclusions contained in the steel, inclusions of a high melting point and a hard type remain as granular substances after hot rolling, and after cold rolling, hard particles crushed to some extent remain in a line in the rolling direction. Therefore, it is thought that if the generation of this type of hard non-metallic inclusion can be remarkably suppressed, the fatigue resistance of the thin plate material can be improved. However, in order to obtain a very high strength level in a strain-induced martensite formation type of steel, it is necessary to contain a large amount of Si exceeding 2% by mass. When the Si content in the steel is so high, suppression of formation of hard non-metallic inclusions becomes very difficult. As disclosed in Patent Document 3, even if the method of adding an Fe-Si alloy having a low Al content is applied without adding Al, it does not lead to stable improvement of the fatigue resistance of the thin plate material by itself.

The present invention is intended to realize a thin plate material having a thickness of 20 to 500 μm having a distribution form of non-metallic inclusions effective for improving fatigue resistance in a strain-induced martensite-forming type stainless steel with a high Si content at a mass production site. will be.

As a result of detailed examination, the inventors have found that in a strain-induced martensite formation type high Si-containing stainless steel grade, in order to improve the fatigue resistance of a thin plate spring material, TiN having an equivalent circle diameter of 6.0 μm or more present in a hot-rolled steel sheet is used. It has been found that it is very effective to reduce the amount of spinel-based inclusions and spinel-based inclusions containing at least one of Al and Mg. In addition, the reduction of such coarse inclusions can be achieved by strictly controlling the contamination of the molten steel container, additives, and admixture of Ti and Al from crude materials, and the basicity of the final slag formed after Si addition. It was found that it is feasible in mass production operation by controlling in a lower and narrower range than usual. Then, when this hot-rolled steel sheet is made into a thin sheet in a cold rolling process, inclusions inside the thin sheet become a very advantageous existence form for improving the fatigue resistance. The present invention is based on these findings.

That is, in order to achieve the above object, in the present invention, in mass%, C: 0.010 to 0.200%, Si: more than 2.00% and 4.00% or less, Mn: 0.01 to 3.00%, Ni: 3.00% or more and less than 10.00%, Cr : 11.00 to 20.00%, N: 0.010 to 0.200%, Mo: 0 to 3.00%, Cu: 0 to 1.00%, Ti: 0 to 0.008%, Al: 0 to 0.008%, balance consisting of Fe and unavoidable impurities In a cross section (L cross section) parallel to the rolling direction and the sheet thickness direction, the distance between particles in the rolling direction is 20 μm or less (ie, 0 to 20 μm), and the distance between particles in the sheet thickness direction is 10 μm or less (ie, 0 to 10 μm), the number density of non-metallic inclusions having a length in the rolling direction of 40 μm or more is 3.0 pieces/mm 2 or less in the L cross section, when regarded as one non-metal inclusion, sheet thickness of 20 to 500 ㎛ stainless steel sheet is provided.

Here, the distance (μm) between grains in the rolling direction for any two grains shown in the L cross section is, when there is no overlap in the rolling direction ranges in which the respective grains exist, the rolling direction distance between the two rolling direction ranges ( μm), and when there is an overlap, it is set to 0 μm. Similarly, the distance (µm) between the two grains in the thickness direction for any two particles shown in the L cross section is, when there is no overlap in the thickness direction ranges in which the respective particles exist, between the two thickness direction ranges. It is set as thickness direction spacing (micrometer), and when there is an overlap, it is set as 0 micrometer. Two particles having an inter-particle distance of 20 µm or less in the rolling direction and an inter-particle distance of 10 µm or less in the sheet thickness direction belong to the same "group".

Among the non-metallic inclusions having a length of 40 μm or more in the rolling direction, the influence on the fatigue resistance is particularly large, and among (i) TiN-based inclusion particles and (ii) spinel-based inclusion particles containing one or more of Al and Mg, the above Those containing one or two of (i) and (ii) are exemplified.

The above Ti content is the total Ti content in the steel including Ti present as an inclusion. Likewise, the above Al content is the total Al content in the steel including Al present as inclusions.

The number density of inclusions can be measured by observing the observation surface obtained by mirror polishing the L cross section with a scanning electron microscope (SEM). Determination of whether the type of inclusion is a TiN-based inclusion or a spinel-based inclusion containing one or more of Al and Mg is, for example, elemental analysis using EDX (energy dispersive fluorescence X-ray spectrometer) attached to the SEM can be done by

Fig. 1 illustrates an SEM photograph of inclusions seen in the L cross section of a conventional cold-rolled steel sheet (Conventional Example No. 1 described later) having a sheet thickness of 120 µm. The horizontal direction in the drawing coincides with the rolling direction, and the longitudinal direction coincides with the sheet thickness direction. Groups of non-metallic inclusion particles lined up almost along the rolling direction are seen in two places (A) and (B). The inter-particle distance in the sheet thickness direction between the nearest particles of groups (A) and (B) is indicated by the symbol S in FIG. Since the distance S between grains in the sheet thickness direction exceeds 10 µm, when all grains of groups (A) and (B) are targeted, these grains are "distance between grains in the rolling direction of 20 µm or less, and It does not correspond to a group of non-metallic inclusion particles lined up with a distance between particles in the thickness direction of 10 µm or less. If only the particles of group (A) are taken as an object, all of the constituent particles have an interparticle distance of 20 μm or less in the rolling direction and a distance between the particles in the sheet thickness direction of 10 μm in positional relationship with at least one other constituent particle. Therefore, each particle constituting the group (A) corresponds to "a group of non-metallic inclusion particles arranged in a row with a distance between particles of 20 μm or less in the rolling direction and a distance between particles of 10 μm or less in the sheet thickness direction". Therefore, each particle constituting group (A) is regarded as one non-metallic inclusion. Similarly, each particle constituting group (B) is also regarded as one non-metallic inclusion. In FIG. 1, two non-metal inclusions exist, and the lengths of each in the rolling direction are indicated by _LA and _LB in FIG. Among these, since L _A is 40 μm or more, among these two non-metal inclusions, the non-metal inclusion having a rolling direction length L _A corresponds to “a non-metal inclusion having a rolling direction length of 40 μm or more”.

As a result of EDX analysis, these non-metallic inclusions were all TiN-based inclusions.

Fig. 2 illustrates an SEM photograph of inclusions viewed in a different view from Fig. 1 in the L cross section of a cold-rolled steel sheet having a sheet thickness of 120 µm according to the present invention (Inventive Example No. 5 described later). The horizontal direction in the drawing coincides with the rolling direction, and the vertical direction coincides with the plate thickness direction. Each of the non-metallic inclusion particles arranged in a row in Fig. 2 corresponds to "a group of non-metallic inclusion particles with a distance between the particles in the rolling direction of 20 μm or less and a distance between the particles in the thickness direction of 10 μm or less", so these are considered as one non-metallic inclusion. Since the length of this non-metallic inclusion in the rolling direction slightly exceeds 40 µm, it corresponds to "a non-metallic inclusion having a length of 40 µm or more in the rolling direction".

As a result of EDX analysis, these non-metallic inclusions were TiN-based inclusions.

The number density of non-metallic inclusions having a length of 40 μm or more in the rolling direction in the cross section of the steel sheet L can be obtained as follows.

[Method for measuring the number density of non-metallic inclusions having a length of 40 µm or more in the rolling direction]

SEM observation was performed on the observation surface obtained by mirror polishing the cross section (L cross section) parallel to the rolling direction and the plate thickness direction of the steel plate, and the measurement area in which the length in the rolling direction was 100 μm or more and the length in the thickness direction was the total thickness of the plate. is randomly determined, and among all "non-metallic inclusions having a length in the rolling direction of 40 μm or more" existing in whole or in part in the measurement area, all of the inclusions exist in the measurement area, and a part of the inclusions are included in the measurement area. The number of inclusions protruding out of the region but having a portion of 1/2 or more of the length in the rolling direction exist in the measurement region is counted. This operation is performed for one or two or more non-overlapping measurement areas until the total area of the measurement area is 10 mm2 or more, and the total sum of the counts in each measurement area is divided by the total area of the measurement area is defined as "the number density of non-metallic inclusions having a length in the rolling direction of 40 μm or more (pcs/mm 2 )".

Further, in the stage of hot-rolled steel sheet, it is preferable that the total number density of TiN-based inclusions having an equivalent circle diameter of 6.0 µm or more and spinel-based inclusions containing one or more of Al and Mg in the L cross section is 0.05 pieces/mm2 or less. .

The equivalent circle diameter is a particle diameter converted to the diameter of a circle having the same area as the projected area of the inclusion particle appearing on the observation surface. The equivalent circle diameter of each inclusion particle can be calculated, for example, by computer image processing of an SEM image of the inclusion. The number density of the inclusions in a hot-rolled steel sheet can be obtained as follows.

[Measurement method of number density of inclusions in hot-rolled steel sheet]

SEM observation was performed on the observation surface obtained by mirror polishing the cross section (L cross section) parallel to the rolling direction and the plate thickness direction of the steel plate, determining a rectangular measurement area within a randomly selected field of view, and observing within the field of view, the TiN system. Inclusions or spinel-based inclusions containing at least one of Al and Mg, and among all inclusion particles having an equivalent circle diameter of 6.0 µm or more, the entirety of the particles existing within the measurement region, and a part of the particles are the above-described inclusions. The number of particles protruding out of the measurement area but with a portion of 1/2 or more of the particle area existing within the measurement area is counted. This operation is performed for a plurality of non-overlapping views until the total area of the measurement area is 200 mm 2 or more, and the value obtained by dividing the total sum of the counts in each view by the total area of the measurement area is obtained as a “circle” Total number density of TiN-based inclusions having an equivalent diameter of 6.0 μm or more and spinel-based inclusions containing at least one of Al and Mg (piece/mm 2 )”.

If the above distribution of inclusions is shown in the hot-rolled steel sheet, when it is then subjected to cold rolling to obtain a thin plate material, the above predetermined distribution of inclusions is obtained, and a significant improvement effect in fatigue resistance is obtained. As such a thin plate material, those having a tensile strength of 2000 N/mm 2 or more in the rolling direction are particularly suitable. In a steel material obtained by cold rolling a hot-rolled steel sheet having the above composition, the matrix (metal substrate) is a mixed structure of strain-induced martensite phase and austenite phase.

As a method for manufacturing the steel sheet, additives and auxiliary materials are added to molten steel having a C content of 0.20% or less and having Cr oxide-containing slag on the molten surface, which has undergone a decarburization process of blowing oxygen into Cr-containing molten iron. When performing component adjustment , The molten steel container used, auxiliary raw materials, and auxiliary materials are selected so that the Ti content in the molten steel is 0.008 mass% or less and the Al content is 0.008 mass% or less, and at least Fe-Si alloy is dissolved in the molten steel as an auxiliary material to deoxidize and slag While carrying out the reduction recovery of heavy Cr into molten steel and adjusting the Si content in steel, the slag basicity (CaO/SiO ₂ mass ratio) to 1.3 to 1.5 to obtain molten steel having the above chemical composition;

A step of obtaining a cast steel by casting the molten steel obtained in the above step;

A step of subjecting the cast steel to hot working including at least hot rolling to obtain a hot-rolled steel sheet;

A process of making a cold-rolled steel sheet having a thickness of 20 to 500 μm by performing annealing and cold rolling on the hot-rolled steel sheet one or more times

A method for producing a stainless steel sheet having a is provided.

Here, “Selecting the molten steel container, supplementary raw materials, and additives to be used so that the Ti content in the molten steel is 0.008 mass% or less and the Al content is 0.008 mass% or less” means, A molten steel container with little or no deposits is used so that the Ti content in the molten steel does not exceed 0.008% by mass or the Al content does not exceed 0.008% by mass due to mixing of Ti and Al in the molten steel. It means using supplementary raw materials or preparations whose impurity content is controlled to be low. Molten steel at the stage where the decarburization process of blowing oxygen into Cr-containing molten iron can be regarded as having substantially zero content of Ti and Al. Therefore, by preventing or reducing contamination from the outside as much as possible, steel having a Ti content of 0 to 0.008% and an Al content of 0 to 0.008% can be obtained.

It is more preferable to select the molten steel container used, additives, and preparations so that the Ti content in the molten steel is 0.006 mass% or less and the Al content is 0.006 mass% or less.

Examples of the molten steel container include a refinery container and a ladle lined with a refractory material. The ladle can also be used as a scouring vessel as it is. As the container for accommodating molten steel, it is preferable to use a container (new pot) in which the refractory material constituting the inner surface of the container has not yet been used for accommodating the molten steel.

It is preferable to use what is 0.05 mass % or less of Al content, and 0.05 mass % or less of Ti content as said Fe-Si alloy.

In addition, the content of Mg, which is a spinel-based inclusion forming element, is not particularly specified in the steel, but as described above, by selecting an effective molten steel container for reducing the content of Ti and Al, supplementary materials, and crude materials, problems can be solved. It has been confirmed that it can be reduced to a level that is not present. In this case, the total Mg content in the steel is 0.002% by mass or less.

By subjecting the cold-rolled steel sheet to an aging treatment, a steel sheet having a matrix (metal substrate) of a mixed structure of strain-induced martensite phase and austenite phase and a tensile strength in the rolling direction of, for example, 2000 N/mm 2 or more can be obtained.

According to the present invention, in a strain-induced martensite formation type high Si-containing stainless steel grade, it is possible to realize a thin plate in which the number of hard non-metallic inclusions having a long rolling direction is significantly reduced in mass production operation. This type of high-Si steel can express the highest level of strength among stainless steels, and has been mainly used for applications such as ID saw blades. Since the inclusion control according to the present invention improves the fatigue resistance of thin plate materials, it is possible to apply it to the use of thin plate spring materials. Therefore, the present invention can contribute to further miniaturization of thin plate spring parts used in electronic devices and other devices by utilizing the high-strength characteristics peculiar to this steel type.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a SEM photograph of non-metallic inclusions seen in the L cross section of a cold-rolled steel sheet of Conventional Example No. 1.
2 is a SEM photograph of non-metallic inclusions seen in the L cross section of the cold-rolled steel sheet of Example No. 5 of the present invention.
Figure 3 is a SEM photograph of typical TiN-based inclusions observed in the cross section of the hot-rolled steel sheet L of Conventional Example No. 4.
Figure 4 is a SEM photograph of typical TiN-based inclusions observed in the cross section of the hot-rolled steel sheet L of Example No. 5 of the present invention.

[chemical composition]

Hereinafter, "%" regarding chemical composition means "mass %" unless otherwise specified.

In the present invention, steel having a chemical composition shown in the following (A) is intended.

(A) In mass%, C: 0.010 to 0.200%, Si: more than 2.00% and 4.00% or less, Mn: 0.01 to 3.00%, Ni: 3.00% or more and less than 10.00%, Cr: 11.00 to 20.00%, N: 0.010 to 0.010% 0.200%, Mo: 0 to 3.00%, Cu: 0 to 1.00%, Ti: 0 to 0.008%, Al: 0 to 0.008%, balance Fe and unavoidable impurities.

A steel grade having this composition produces strain-induced martensite during cold rolling, and its strength increases. In addition, by the subsequent aging treatment, solute atoms such as C and N form a Cottrell atmosphere mainly in the martensite phase to fix dislocations and improve strength (strain aging). In addition, Si, which is present in a large amount in steel, causes solid solution strengthening of martensite phase or retained austenite phase, and contributes to strength improvement.

In the present invention, in order to fully enjoy the above-described strength-improving effect of Si, in particular, those with a Si content of more than 2.00% are targeted. However, when the Si content is too large, adverse effects such as easy occurrence of hot working cracks become apparent. Here, the Si content is limited to 4.00% or less.

C is an austenite phase forming element and is also an element necessary for strengthening steel. However, excessive C content causes deterioration in corrosion resistance and toughness. In the present invention, steels with a C content of 0.010 to 0.200% are targeted, but in particular when high strength is aimed at, it becomes advantageous to set the C content in the range of 0.050 to 0.100.

N is an austenite phase forming element and is also an element necessary for strengthening steel. However, excessive content becomes a factor that promotes the formation of TiN-based inclusions. In the present invention, steel with an N content of 0.010 to 0.200% is targeted. Within this range, it is possible to optimize the particle size distribution of TiN-based inclusions within the range stipulated in the present invention by a production method that suppresses Ti incorporation as described later. A more preferable range of the N content is 0.050 to 0.085%.

Mn is an element whose austenite stability can be easily controlled by adjusting its content, and its content is adjusted in the range of 0.01 to 3.00%. When a large amount of Mn is contained, it becomes difficult to induce deformation induced martensite phase. It is more preferable to adjust the Mn content in the range of 1.00% or less, and you may manage it in the range of 0.50% or less.

Ni is an austenite phase forming element, and a content of 3.00% or more is secured to form a metastable austenite phase at room temperature. When the Ni content is too large, the deformation induced martensite phase becomes difficult to be induced, so the Ni content is made less than 10.00%. It is more preferable to set it as 7.00 to 9.50%.

Cr is an element required to ensure corrosion resistance. In the present invention, steel having a Cr content of 11.00 to 20.00% is targeted. Cr is a ferrite phase forming element, and when it is contained in a large amount exceeding the above, an austenite single phase structure may not be obtained at high temperatures. A more preferable range of the Cr content is 12.00 to 15.00%.

Mo, in addition to having an action of improving corrosion resistance, contributes to strengthening by forming Mo-based precipitates by aging treatment, and has an action of making it difficult for the tissue work-hardened by cold rolling to be softened by aging treatment, It can be included as needed. In order to fully enjoy these actions, it is preferable to secure a Mo content of 1.0% or more. Further, when a strength level such that the tensile strength in the rolling direction is 2000 N/mm 2 or more is intended, it is extremely effective to contain 2.00% or more of Mo. However, since a large amount of Mo content causes generation of a δ ferrite phase at high temperature, when Mo is contained, the content range is 3.00% or less. You may manage in the range of 2.50% or less.

Since Cu has an action of increasing strength by interaction with Si during aging treatment, it can be contained as necessary. In that case, it is more preferable to set it as 0.01% or more of Cu content. A large amount of Cu is a factor in reducing hot workability. When containing Cu, it is set as 1.00% or less of content range.

Ti is an element that forms TiN-based inclusions, and since TiN is easily formed particularly in high Si-containing steel, it is necessary to keep the Ti content low. As a result of various studies, the Ti content needs to be 0.008% or less, more preferably 0.006% by mass or less. The lower the Ti content, the better, but in mass production operations, it is reasonable to set the Ti content in the range of 0.001% or more in consideration of cost.

Al forms Al ₂ O ₃ and causes spinel-based inclusions. Since Al ₂ O ₃ tends to be formed in molten steel especially in steels containing high Si, it is necessary to keep the Al content low. In the present invention, the lower the Al content, the better. As a result of various studies, the Al content needs to be 0.008% or less, more preferably 0.006% by mass or less. The lower the Al content, the better, but in mass production operations, it is reasonable to set the Al content in the range of 0.001% or more in view of cost. However, even if the Al content is in the above range, it is difficult to stably obtain the particle size distribution of spinel-based inclusions within the range specified in the present invention unless the slag basicity after Si addition is appropriated as described later.

As an unavoidable impurity, the P content is preferably 0.040% or less, the S content is preferably 0.002% or less, and the Mg content is preferably 0 to 0.002%.

In order to adjust the ease of formation of the deformability induced martensite phase in cold rolling, it is preferable that the Md ₃₀ value defined by the following formula (1) is in the range of -50 to 0.

Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo... (One)

Here, the value of the content of the element expressed in terms of mass% is substituted for the element symbol of formula (1).

[Non-metallic inclusions]

Non-metallic inclusions present in steel are roughly divided into soft types with a low melting point and hard types with a high melting point. In the case of the subject steel of the present invention, the former soft type is mainly of the CaO-SiO ₂ system. Since this soft type inclusion is liquid at the hot rolling temperature, it is elongated in the rolling direction during hot rolling, and in subsequent cold rolling, it is crushed and dispersed more finely. Soft inclusions of this kind hardly adversely affect the fatigue resistance properties of thin plates.

It is the inclusions of the latter hard type that are problematic. Inclusions of this kind remain as granular substances after hot rolling, and after cold rolling, hard particles crushed to some extent remain in the form of lined up in the rolling direction. As the plate thickness becomes thinner, the ratio of the length of the inclusions to the plate thickness in the plate thickness direction increases, and stress is concentrated around the inclusion particles, making it easier to function as the origin or propagation path of cracks. It was found that the hard-type inclusions that are problematic in the subject steel of the present invention are TiN-based inclusions and spinel-based inclusions containing at least one of Al and Mg. In particular, TiN-based inclusions tend to grow as the solubility of Ti decreases in the process of lowering the temperature of the molten steel during casting, which tends to cause problems.

According to the study of the inventors, at the stage of hot-rolled steel sheet, among the TiN-based inclusions and spinel-based inclusions, the circle equivalent diameter is 6.0 μm or more, but if the number ratio is small, for example, a thin plate material such as a sheet thickness of 20 to 500 μm When , an advantageous inclusion distribution form is obtained to improve endothelial properties when subjected to repeated elastic deformation. Specifically, at the stage of a hot-rolled steel sheet, TiN-based inclusions having an equivalent circle diameter of 6.0 µm or more and spinel-based inclusions containing one or more of Al and Mg in a cross section (L cross section) parallel to the rolling direction and sheet thickness direction It is very effective to set the total number density of 0.05 pieces/mm 2 or less in a tissue state.

As a more preferable microstructure state of the hot-rolled steel sheet, in addition to the above rule of number density, the maximum by the equivalent circle diameter of TiN-based inclusions and spinel-based inclusions containing at least one of Al and Mg in the L cross section described above. A metal structure having a particle diameter of 10.0 μm or less is exemplified. In this case, the L cross-section measurement area for determining the maximum particle diameter may be 200 mm 2 or more.

In a thin plate having a thickness of 20 to 500 µm, the number density of non-metallic inclusions having a length in the rolling direction of 40 µm or more is 3.0 pieces/mm2 or less in the L cross section in order to improve fatigue resistance when subjected to repeated elastic deformation. found to be extremely effective. Here, as described above, a group of non-metallic inclusion particles aligned with a distance of 20 μm or less in the rolling direction and a distance of 10 μm or less in the sheet thickness direction are regarded as one non-metal inclusion.

Regarding the fatigue resistance, adjacent inclusion particles lined up and approaching to a certain extent function as a starting point for crack generation, similarly to the case where they exist as one continuous particle. As a result of various investigations, a particle group consisting of a plurality of non-metallic inclusion particles (considered as one non-metal inclusion) arranged side by side while maintaining a distance of 20 μm or less between particles in the rolling direction and 10 μm or less between particles in the sheet thickness direction. In particular, those having a length in the rolling direction of 40 μm or more tend to be the starting point of crack generation when repeated elastic deformation is applied to the high-strength steel targeted in the present invention. However, even with such non-metallic inclusions, if the number density in the L cross section is reduced to 3.0 pieces/mm 2 or less, the fatigue resistance is improved. As a reason for this, it is estimated that when the density of non-metallic inclusions having a length of 40 μm or more in the rolling direction is sufficiently reduced, these non-metallic inclusions may become difficult to exhibit their function as a crack propagation path.

The lower the number density at the L cross section of non-metallic inclusions having a length of 40 µm or more in the rolling direction, the more favorable the improvement in the fatigue resistance of the thin plate is. If, for example, steel is melted in an experimental melting furnace or the like using only high-purity raw materials without using scrap, it is considered possible to produce a thin plate material with very little non-metallic inclusions. However, when a steel sheet having a thickness of 20 to 500 μm is manufactured at a mass production site, completely preventing the generation of non-metallic inclusions having a length of 40 μm or more in the rolling direction increases the load on the steelmaking process, leading to an increase in cost. Therefore, in a thin plate having a sheet thickness of 20 to 500 µm, it is reasonable to set the number density in the L cross section of non-metallic inclusions having a length in the rolling direction of 40 µm or more in the range of 0.1 to 3.0 pieces/mm 2 .

[Manufacturing method]

The stainless steel sheet in which the particle size distribution of the hard nonmetallic inclusions described above is optimized can be manufactured using a general stainless steel melting facility. Representative examples include a VOD process and an AOD process. In either case, first, molten steel with a C content of 0.20% or less having Cr oxide-containing slag on the molten surface after a decarburization process of blowing oxygen into Cr-containing molten iron is manufactured. Up to this stage, the steelmaking process can proceed according to a conventional method, except for selecting a container in which little or no Ti or Al is mixed from attachments or the like as the molten steel container to be used.

Since the molten steel at this stage is decarburized molten steel that blows in oxygen, almost all of the oxide dioxide elements Ti, Al, Mg, and Si are oxidized and removed from the molten steel. That is, Ti, Al, Mg, and Si hardly exist in molten steel. In addition, Cr contained in a large amount in molten steel is also partially oxidized to form slag on the molten steel surface as Cr oxide. In this Cr oxide-based slag, Ti, Al, Mg, and Si removed from molten steel also exist as oxides. On the other hand, a large amount of oxygen blown in for decarburization is dissolved in molten steel. Therefore, it is necessary to perform deoxidation before casting. Moreover, in this invention, in order to manufacture high-Si steel whose Si content exceeds 2.00%, it is necessary to contain Si in steel. In addition, it is preferable to carry out a treatment (reduction and recovery of Cr) to return Cr from the molten steel to the steel from the slag during decarburization. Therefore, in the present invention, the above-mentioned "deoxidation", "Si content adjustment", and "Cr reduction recovery" are performed at once by introducing the Fe-Si alloy into molten steel. In addition, if necessary, other auxiliary materials are introduced to adjust the components.

When an Fe-Si alloy is injected into molten steel and the Si content is adjusted to exceed 2.00%, deoxidation in the steel proceeds due to the Si source added in a large amount. The oxygen concentration in the steel by this deoxidation is determined by the chemical equilibrium in the chemical reaction of the following formula (2).

Si(in metal)+2O(in metal)=SiO ₂ (in slag) … (2)

This equilibrium constant K is represented by the following formula (3).

K=A(SiO ₂ )/A(Si)/A(O) ² . . . (3)

Here, A(X) is the activity of component X. As can be seen from the equation (3), the higher the Si activity (ie, Si concentration) in molten steel, the lower the oxygen activity (ie, oxygen concentration) in molten steel. Therefore, in the molten steel of the subject of the present invention to which a large amount of Si source is added, the oxygen concentration in the molten steel is lower than in the case of low Si-containing steel (for example, general steel such as SUS304).

On the other hand, chemical equilibrium based on the following formula (4) is also established between Al oxide in slag and oxygen in molten steel.

2Al (in metal) + 3O (in metal) = Al ₂ O ₃ (in slag) … (4)

By this chemical equilibrium, when the oxygen concentration in molten steel is low, the equilibrium is maintained by increasing the Al concentration in molten steel. This relationship applies equally to Ti or Mg. That is, when the oxygen concentration in molten steel is low, the Al concentration, Ti concentration, and Mg concentration in molten steel increase.

The higher the Al concentration and Mg concentration in molten steel, the easier it is to generate and grow spinel-based inclusions. The higher the Ti concentration in molten steel, the easier it is to generate and grow TiN-based inclusions. Therefore, in order to suppress the generation and growth of these inclusions, it is necessary to suppress the decrease in the oxygen concentration in the molten steel due to the increase in the Si concentration in the molten steel as much as possible. In order to suppress the fall of the oxygen concentration in molten steel, it is more advantageous that the _SiO2 concentration in slag is higher. So, in this invention, the method of controlling slag basicity (CaO/ _SiO2 mass ratio) low is taken. Specifically, the input amount of the Ca-containing substance added as an auxiliary agent is adjusted. It is good to use quicklime CaO as an auxiliary agent. Moreover, Ca is supplied in slag also from _CaF2 added as a flux component as needed. Ca supplied from CaF ₂ is also converted into the amount of CaO and added to the CaO value at the time of calculating the basicity. As a result of various investigations, it was found that it is effective to make the slag basicity within the range of 1.3 or more and 1.5 or less in the slag existing on the molten surface after the introduction of the Fe-Si alloy is finished. It is more preferable to set it as 1.3 or more and 1.45 or less, and it is still more preferable to set it as 1.3 or more and 1.4 or less. The lower the basicity of the slag, the lower the oxygen concentration in the molten steel is, and the easier it is for Ti, Al, and Mg to enter the molten steel from the slag. However, when the slag basicity is too low, a large amount of other types of inclusions such as Cr ₂ O ₃ are produced. Also, the desulfurization ability is also lowered. Therefore, it is extremely effective to control the final slag basicity within a narrow range not lower than 1.3.

As described above, Ti, Al, and Mg are hardly contained in molten steel that has undergone oxygen blowing and decarburization, and these elements exist as oxides in slag mainly composed of Cr oxide. Ti, Al, and Mg in this slag are mixed from raw materials and refractories, and from pre-charged slag, metal, and the like attached to facilities such as electric furnaces and converters. In order to optimize the particle diameter distribution of hard inclusions in the steel sheet as described above by controlling the slag basicity, Ti, Al , it is necessary to prevent the new incorporation of Mg as much as possible. In particular, it has been confirmed that when slag or the like adhered to the molten steel container from previous filling remains, coarse hard inclusions are likely to be formed due to a small amount of Ti or Al mixed in from the deposit. In order to prevent mixing from the molten steel container, it is most preferable to use a molten steel container (new pot) in which the refractory material constituting the inner surface of the container has not yet been used to contain the molten steel. In addition, it has been confirmed that impurities such as Al and Ti are contained in Fe—Si alloys generally used in stainless steel manufacturing sites, and Al and Ti mixed therein also cause formation of coarse hard inclusions. Therefore, in the present invention, it is necessary to apply a high-purity Fe-Si alloy. Specifically, it is preferable to use an Fe-Si alloy having an Al content of 0.05 mass% or less and a Ti content of 0.05 mass% or less. It is also desirable to take care not to mix Ti and Al from other additives and auxiliary materials as much as possible.

Finally, it is important to select the molten steel container, auxiliary raw materials, and preparations so that the Ti content in the molten steel is 0.008 mass% or less and the Al content is 0.008 mass% or less. When the Ti content and Al content in the final steel exceed the above, it is difficult to stably realize the desired hard inclusion particle diameter distribution even if the above-described control of the slag basicity is performed. In addition, it is desirable to control the content of Mg in molten steel to be 0.002% by mass or less in the end. It was confirmed that the particle diameter distribution of the spinel-based inclusions was in the above-described desired state, and no problem occurred, even if the weight content was not particularly regulated.

It is more preferable to set the Ti content in molten steel to 0.006 mass% or less and the Al content to 0.006 mass% or less.

Casting may be performed according to a conventional method. Usually, a cast steel is obtained by a continuous casting method. In this specification, steel materials obtained by casting (those having a solidification structure) are referred to as cast steel. Therefore, ingots (ingots) obtained by the ingot method are also included in the cast steel referred to here for convenience.

The obtained cast steel is subjected to hot working including at least hot rolling to obtain a hot-rolled steel sheet. In the case of the ingot method, hot rolling is performed after passing through a lump rolling or hot forging. The heating temperature of the hot rolling may be 1100 to 1250°C, and the thickness of the hot-rolled steel sheet may be, for example, 2.5 to 6.0 mm. In this way, in the cross section (L cross section) parallel to the rolling direction and the sheet thickness direction, the total number density of TiN-based inclusions having an equivalent circle diameter of 6.0 µm or more and spinel-based inclusions containing at least one of Al and Mg is 0.05 /mm2 or less can be obtained a stainless steel hot-rolled steel sheet.

Next, by subjecting this hot-rolled steel sheet to annealing, cold rolling, and aging treatment, a thin sheet material of high-strength stainless steel can be obtained. The cold rolling process may be performed a plurality of times including the intermediate annealing process. After each heat treatment process, pickling is performed as needed. The conditions of annealing (hot-rolled sheet annealing) given to a hot-rolled steel sheet are, for example, 1000 to 1100 ° C × 40 to 120 sec, and the final cold rolling rate (when performing intermediate annealing, the cold rolling rate after the final intermediate annealing) is an example. For example, 40 to 70%, aging treatment conditions can be, for example, 400 to 600 ° C × 10 to 60 min. In the application of a thin plate spring material, it is preferable to make the final plate thickness into 150 micrometers or less, for example, and it is more preferable to set it as less than 100 micrometers. For example, it can also be made into a thin plate material with a thickness of 20 to 70 μm. In this way, a thin sheet material of high-strength stainless steel in which the matrix (metal substrate) is a mixed structure of strain-induced martensite phase and austenite phase can be obtained. The ratio of the area ratio M of the strain-induced martensite phase to the area ratio A of the austenite phase is usually in the range of M:A from 30:70 to 50:50. In what contains 2.00% or more of Mo, for example, high strength with a tensile strength of 2000 N/mm 2 or more in the rolling direction can be obtained. In a thin plate material, when a group of non-metallic inclusion particles arranged in a row with a distance of 20 μm or less in the rolling direction and a distance between particles in the thickness direction of 10 μm or less is regarded as one non-metal inclusion, the length in the rolling direction is 40 μm or more A texture state in which the number density of non-metallic inclusions is 3.0 pieces/mm 2 or less in the L cross section is obtained, and the thin steel sheet exhibits good fatigue resistance in use as a spring material to which repeated elastic stresses are applied.

Example

The steel shown in Table 1 was smelted by the VOD process. After completing the final decarburization process of blowing oxygen into Cr molten iron with VOD facilities, molten steel having a C content of 0.10% or less with Cr oxide-containing slag on the molten surface was obtained. The C content at this stage is almost equal to the final C content shown in Table 1.

In the final decarburization in the VOD facility, a ladle was used as a molten steel container, and then the process before casting was performed with the same ladle. In the conventional examples of Nos. 1 and 2, the ladle was used for melting Ti-containing stainless steel as a pre-charge, and in the conventional examples of Nos. 3 and 4, the ladle used for solvent-free stainless steel without Ti was used as a pre-charge, and No. 5 In the example of the present invention, a refractory material constituting the inner surface of the ladle was not yet used for accommodating molten steel (new pot).

An Fe-Si alloy was added to the molten steel after the final decarburization, and the Si content in the molten steel was adjusted to a target value, while deoxidation and reduction and recovery of Cr in the slag were performed. The Si content in the molten steel at this stage is almost equal to the final Si content shown in Table 1. As the Fe-Si alloy, in the conventional examples of Nos. 1 to 4, an alloy corresponding to ferrosilicon No. 2 specified in JIS G2302:1998 was used. As a result of the analysis, this ferrosilicon product No. 2 contained Al: about 1.0% by mass, Mg: about 0.07% by mass, and Ti: about 0.08% by mass, although there was some variation depending on the product lot. On the other hand, in the example of the present invention of No. 5, a high-quality Fe-Si alloy having an extremely reduced Al content was used. As a result of the analysis, the Al, Mg, and Ti contents of this high-grade Fe-Si alloy were Al: 0.009% by mass, Mg: less than 0.001% by mass, and Ti: 0.012% by mass.

Following the addition of the Fe-Si alloy, industrial quicklime (CaO) was introduced into the slag as a roughing agent. Thereafter, the slag was collected and component analysis was performed. As a result, the slag basicity was 1.60 to 1.65 in the conventional examples of Nos. 1 to 4 and 1.33 in the present invention example of No. 5.

In each case, the molten steel obtained as described above was continuously cast and subjected to hot rolling to obtain a hot-rolled steel sheet having a thickness of 3.8 mm. The heating temperature in hot rolling was 1230 degreeC. For the obtained hot-rolled steel sheet, the L cross section was observed by SEM, and TiN-based inclusions having an equivalent circle diameter of 6.0 µm or more and Al and Mg were measured according to "Measurement method for number density of inclusions in hot-rolled steel sheet" previously published. The total number density of spinel-based inclusions containing one or more species was measured. As a result, the total number density was 0.20 to 0.45 pieces/mm 2 in the conventional examples of Nos. 1 to 4, and 0.02 pieces/mm 2 in the present invention example of No. 5. It was found that the generation of coarse hard inclusions can be remarkably suppressed by the solvent method according to the present invention.

FIG. 3 shows a SEM photograph of typical TiN-based inclusions observed in the L cross section of the hot-rolled steel sheet of Conventional Example No. 4, and FIG. foreshadow The horizontal direction of all photographs is the rolling direction. In addition, the cross cursor shown in the picture indicates the beam irradiation position of EDX analysis.

Next, using the sample taken from the hot-rolled steel sheet, hot-rolled sheet annealing at 1050 ° C. × 60 sec, cold rolling, intermediate annealing at 1050 ° C. × 60 sec, cold rolling, and aging treatment at 500 ° C. × 30 min were performed, and matrix (metal Substrate) was fabricated a thin plate material with a plate thickness of 120㎛, which is a mixed structure of the processing induced martensite phase and the austenite phase. Rolling of the thin plate material obtained

The tensile strengths in all directions exceeded 2000 N/mm 2 .

For the L cross section of this thin plate material, the number density of non-metallic inclusions having a length of 40 μm or more in the rolling direction was measured according to “Method for Measuring Number Density of Non-Metal Inclusions with a Length of 40 μm or More in the Rolling Direction” previously published. However, as described above, a group of non-metallic inclusion particles aligned with a distance of 20 μm or less in the rolling direction and a distance of 10 μm or less in the sheet thickness direction was regarded as one non-metal inclusion. As a result of the measurement, the number density in the L cross section of the non-metallic inclusions having a length of 40 µm or more in the rolling direction was 8.2 to 33.2 pieces/mm2 in the conventional examples of Nos. 1 to 4 and 2.4 pieces/mm2 in the inventive example of No. 5. As a result of the EDX analysis, the non-metallic inclusions to be counted were composed of TiN-based inclusion particles or spinel-based inclusion particles containing at least one of Al and Mg.

In the examples of the present invention, the number of hard non-metallic inclusions having a length of 40 μm or more in the rolling direction, which is a factor for deterioration of the fatigue resistance, was significantly reduced compared to the conventional examples.

For reference, a manufacturing example in the case of performing deoxidation by Fe—Si alloy for SUS304 (Si content of 0.55%) is shown. The ladle used for the melting of Ti-containing stainless steel in the previous charge is used as a molten steel container, and the decarburization process of blowing oxygen into Cr molten iron as a VOD facility is completed, and the molten steel with a C content of about 0.05% has Cr oxide-containing slag on the molten surface. was obtained. To this molten steel, an Fe—Si alloy equivalent to the above Ferro Silicon No. 2 was added to adjust the Si content. In addition, industrial quicklime (CaO) as an auxiliary agent was added. After that, chromium nitride was added to complete component adjustment. As a result of collecting and analyzing the final slag, the basicity of the slag was 1.65. This molten steel was continuously cast and hot-rolled in a conventional manner to obtain a hot-rolled steel sheet having a thickness of 3.5 mm. Regarding this hot-rolled steel sheet, the presence condition of non-metallic inclusions was investigated in the same manner as in the case of Nos. 1 to 5 above. As a result, TiN-based inclusions having an equivalent circle diameter of 6.0 µm or more and spinel-based inclusions containing at least one of Al and Mg were not observed. Comparing this example of SUS304 with the conventional examples of Nos. 1 to 4 described above, it is found that it is very difficult to suppress the formation of hard inclusions in stainless steels with a high Si content.

Claims (9)

In mass%, C: 0.010 to 0.200%, Si: more than 2.00% and 4.00% or less, Mn: 0.01 to 3.00%, Ni: 3.00% or more and less than 10.00%, Cr: 11.00 to 20.00%, N: 0.010 to 0.200%, Mo: 0 to 3.00%, Cu: 0 to 1.00%, Ti: 0 to 0.008%, Al: 0 to 0.008%, balance Fe and unavoidable impurities, having a cross section parallel to the rolling direction and sheet thickness direction (L cross section), when a group of non-metallic inclusion particles arranged in a row with a distance between particles in the rolling direction of 20 μm or less and a distance between particles in the thickness direction of 10 μm or less is regarded as one non-metal inclusion, the length in the rolling direction is 40 μm. A stainless steel sheet having a sheet thickness of 20 to 500 μm, wherein the number density of non-metallic inclusions of μm or more is 3.0 pieces/mm 2 or less in the L cross section. The non-metal inclusions according to claim 1, wherein the non-metallic inclusions having a length in the rolling direction of 40 μm or more are (i) TiN-based inclusion particles, (ii) spinel-based inclusion particles containing at least one of Al and Mg, (i), A stainless steel sheet containing one or two of (ii). The stainless steel sheet according to claim 1 or 2, wherein the tensile strength in the rolling direction is 2000 N/mm 2 or more. The stainless steel sheet according to claim 1 or 2, wherein the matrix (metal substrate) is a mixed structure of strain-induced martensite phase and austenite phase. When component adjustment is performed by adding additives and auxiliary materials to molten steel having a C content of 0.20% or less and having Cr oxide-containing slag on the molten surface, which has undergone the decarburization process of blowing oxygen into Cr-containing molten iron, the Ti content in the molten steel is 0.008 The molten steel container to be used, additives, and preparations are selected so that the Al content is 0.008% by mass or less, and at least Fe-Si alloy as additives is dissolved in molten steel for deoxidation and reduction of Cr in slag into molten steel. While recovering and adjusting the Si content in the steel, adding a Ca-containing auxiliary agent to adjust the slag basicity (CaO/SiO ₂ mass ratio) to 1.3 to 1.5, to obtain molten steel having the chemical composition shown in (A) below process to do,
A step of obtaining a cast steel by casting the molten steel obtained in the above step;
A step of subjecting the cast steel to hot working including at least hot rolling to obtain a hot-rolled steel sheet;
A process of making a cold-rolled steel sheet having a thickness of 20 to 500 μm by performing annealing and cold rolling on the hot-rolled steel sheet one or more times
Method for producing a stainless steel sheet having a.
(A) In mass%, C: 0.010 to 0.200%, Si: more than 2.00% and 4.00% or less, Mn: 0.01 to 3.00%, Ni: 3.00% or more and less than 10.00%, Cr: 11.00 to 20.00%, N: 0.010 to 0.010% 0.200%, Mo: 0 to 3.00%, Cu: 0 to 1.00%, Ti: 0 to 0.008%, Al: 0 to 0.008%, balance Fe and unavoidable impurities. The method for manufacturing a stainless steel sheet according to claim 5, wherein a refractory material constituting the inner surface of the container is used as the molten steel container (a new pot) that has not yet been used to contain the molten steel. The method for producing a stainless steel sheet according to claim 5, wherein, as the Fe-Si alloy, an Al content of 0.05% by mass or less and a Ti content of 0.05% by mass or less are used. The method for producing a stainless steel sheet according to claim 5, further comprising a step of subjecting the cold-rolled steel sheet to an aging treatment. The method for producing a stainless steel sheet according to claim 8, wherein the matrix (metal substrate) is a mixed structure of strain-induced martensite phase and austenite phase, and a steel sheet having a tensile strength in a rolling direction of 2000 N/mm 2 or more is obtained.

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